Complexation to FeII, NiII, and ZnII of multidentate ligands resulting from condensation of 2-pyridinecarboxaldehyde with α,ω-triamines:: Selective imidazoildine/hexahydropyrimidine ring opening revisited

The condensation reaction between 2-pyridinecarboxaldehyde and diethylenetriamine, 3-[(2-aminoethyl)amino]propylamine, and 3,3'-iminobis(propylamine) in a 2:1 molar ratio yields ligands that may be isolated exclusively in the dissymmetric (cyclic) isomeric forms L-A, L-B/L-B*, and L-C. The template effect of a metal center (Fe-parallel to, Ni-parallel to,and Zn-parallel to) results in the ring opening of L-C including one hexahydropyrimidine ring and one (long) propylene bridge. The resulting symmetric bis-Schiff base isomeric form L-C' is stabilized through pentacoordination, yielding ((FeLC')-L-parallel to(NCS)]-(NCS) (3), [(NiLC')-L-parallel to(NCS)](NCS) (6), and [(ZnLC')-L-parallel to(NCS))(NCS) (9). The same metal centers are too bulky to exert a template effect on L A including one imidazolidine ring and one (short) ethylene bridge. LA acts as a tetradentate ligand yielding [(FeLA)-L-parallel to(NCS)(2)] (1), [(NiLA)-L-parallel to(NCS)(2)] (4), and [(ZnLA)-L-parallel to(NCS)(2)] (7). The template effect of the metal center is selective toward the ligand LB/LB* including a hexahydropyrimidine (imidazolidine) ring and the shorter ethylene (longer propylene) bridge. The Fell cation is small enough to exert a template effect, resulting in the ring opening of L-B/L-B*. The resulting bis-Schiff base L-B' is stabilized through pentacoordination, yielding [Fe(parallel to)lL(B')(NCS)](NCS) (2). Ni-parallel to is too bulky to promote the ring opening of L-B/L-B*: L-B acts as a tetradentate ligand, yielding [(NiLB)-L-parallel to(NCS)(2)] (5) (the L-B* isomer is totally converted to L-B). The coordinative requirements and stereochemical preference of the bulkier Zn-parallel to cation allow neither the ring opening of L-B/L-B* nor the tetracoordination of L-B or L-B* but stabilize the novel tetradentate dissymmetric form L-B degrees in [(ZnLB degrees)-L-parallel to(NCS)(2)]-H2O (8) (L-B degrees results from MeOH addition across the imine bond of L-B). Density functional theory calculations performed for Ni (parallel to) and Zn-parallel to complexes of the L-B/L-B*/L-B degrees set of ligands allowed one to compare the relative stabilities of all possible isomers, showing that the most stable ones correspond to those experimentally obtained: isomerization, or methanol addition across the imine bond, of the tetradentate ligand depends on the relative stabilities of all possible isomeric complexes.